1. Field of the Invention
One disclosed aspect of the embodiments relates to a distance measurement apparatus, a distance measurement method, and a camera, and more particularly, to an object distance measurement apparatus and an object distance measurement method for use in a digital still camera or a digital video camera.
2. Description of the Related Art
In a digital still camera or a video camera, U.S. Pat. No. 4,027,113 discusses a solid-state image sensor, in which distance measurement pixels having a distance measurement (focus detection) function are arranged at all or part of pixels of the solid-state image sensor to measure a distance in a phase difference method.
The distance measurement pixel includes a plurality of photoelectric conversion units and is configured such that light flux passing through different exit pupil areas of a imaging lens is guided to different photoelectric conversion units.
By using a plurality of distance measurement pixels, images by light flux passing through different exit pupil areas (image A and image B) are detected, and an image displacement amount of the image A and the image B is measured. A distance (focal position) may be calculated by calculating a defocus amount from the image displacement amount and a base length (interval between different exit pupil areas). In this case, distance accuracy is improved by exactly calculating the image displacement amount and the base length.
Also, United States Patent Publication Application No. 2011/0164165 discusses a method for improving distance accuracy by correcting a sensitivity difference of solid-state image sensors or asymmetry of an image A and an image B and accordingly exactly calculating an image displacement amount.
On the other hand, the base length, which is the interval between different exit pupil areas, is determined based on design values, considering sensitivity of a distance measurement pixel of a solid-state image sensor, a condition of an imaging lens, and vignetting of light flux by a lens frame.
However, the base length is sensitive to the sensitivity of the distance measurement pixel. Thus, if there is a manufacturing error in the solid-state image sensor, a base length error occurs. As a result, distance accuracy is worsened.
Furthermore, in an object that is not a perfect diffuse reflection such as a metal and has a strong angular dependency, light intensity distribution on a pupil is changed. Therefore, a base length is different from a set value. As a result, distance accuracy is insufficient.